This invention relates to a punch-through reference diode, and more particularly to a diode structure capable of preventing the fluctuations of a punch-through voltage and the occurrence of a negative resistance.
Conventional reference diodes utilize the Zener breakdown gain constant voltage characteristics by exploiting the backward characteristics of semiconductor PN-junctions. In a range of low voltages of, for example, below 6 V, however, the operating resistance becomes very great (for example, R.sub.d =1.5k.OMEGA.) in a region of several hundred mA, which has led to the disadvantage that constant voltage characteristics, which are sought, are not attained.
As a device which eliminates this disadvantage, there has been developed a punch-through reference diode which exploits the punch-through effect across the emitter region and collector region of a transistor.
The punch-through reference diode is disclosed in, for example, Japanese Published Unexamined Patent Applications Nos. 49-1183, 53-6582 and 54-14689. The devices shown there make use of an npn-type planar transistor, the collector-base junction of which is punched through in such a way that a reverse bias voltage exceeding a punch-through voltage is applied thereto with the emitter region and base region of the transistor shorted. At this time, the breakdown voltage of the collector-base junction is of course greater than the punch-through voltage.
To be noted in these examples is, in the first place, that the emitter region and base region of the planar transistor are shorted. In the second place, the impurity concentration distribution of the base region is flat, or it has a gradient according to which the impurity concentration is high on the base-emitter junction side and gradually lowers towards the collector-base junction side. That is, the impurity concentration distribution of the base region exhibits such a gradient that the impurity concentration on the shorted junction side is high and that the impurity concentration on the junction side to which the reverse bias voltage is applied is low.
Such prior-art reference diodes have had the disadvantage that, in a region of voltages of above 4 V, abrupt constant-voltage characteristics are not achieved because of the occurrence of a negative resistance. A further disadvantage is that, in a region of currents of below 1 mA, constant voltage characteristics are not attained, so they cannot be used as reference diodes.